The present invention relates to a method of imparting increased foldability to fold lines in paperboard materials for paper containers.
Sealed paper containers are prepared from a paperboard material cut in a predetermined configuration and having a required number of fold lines U-shaped or V-shaped in cross section, by folding the material along the fold lines, adhering together specified portions of the folded material to obtain an open container, sealing off the opening after filling the container with contents, and finally forming the container in shape. The fold lines are formed usually by pressing a fold pattern of ridges against the base paper of the paperboard material using a fold forming device. The paperboard material for sealed containers comprises the base paper and a layer of polyethylene or like thermally bondable material directly formed on each surface of the base paper. When required, aluminum foil or a synthetic resin layer having barrier properties is interposed between one surface of the base paper and the layer of thermally bondable material.
In forming containers from the paperboard material having fold lines, the material is not always accurately bent or folded along the fold line. In actuality, the material is frequently folded off the fold line or with wrinkles. Variations in the folded position or wrinkles not only impair the appearance of the container as a commercial product but also deform the container, possibly causing troubles when such containers are sold by automatic vending machines.